Fatigue Mechanisms of Porous Silicon Carbide under Cyclic Loading(Ceramics & Rocks 1)

Abstract

Notched specimens of porous silicon carbide with porosity 37% were fatigued under four-point bending at frequencies of 30 and 0.3 Hz. The fatigue life expressed in terms of time was rather insensitive to the test frequency, while that expressed in terms of cycles was much shorter for the case of 0.3Hz than for 30 Hz. A time-dependent stress corrosion mechanism was mainly responsible for crack propagation, and stress cycling enhanced the crack propagation mechanism. The crack length was estimated from the change of the compliance of the specimen. The crack propagation curve was divided into Stages I and II. In Stage I, the crack propagation rate decreased even though the applied stress intensity factor got larger with crack extension, and turned to increase in Stage II. The transition from Stage I to II took place at the crack extension of around 0.8 mm. This anomalous behavior is caused by crack-tip shielding due to microcracking and asperity contact. Fractographic observations showed that the fracture path was along the binder phase between SiC particles, more precisely along the interface between particles and binders.